I was afraid of this. The bouncy hump in the middle is an annoying "feature" of the construction process. To be clear, I know it doesn't have real joists, but some of the videos show pieces of wood running lengthwise, with the furnace ductwork nestled exactly in between 2 of those lengthwise pieces. It wouldn't surprise me to learn this entire floor is the foam core sandwich.
I was able to look into the ductwork itself with a borescope to find a section of the duct which is supported/reinforced with aluminum channel in front of the stove...exactly where the hump is the worst. I may put a self tapping screw into that aluminum before covering the floor w vinyl plank...or I may just be annoyed by it every time I walk in.
Thanks for the replies.